Not applicable.
Not applicable.
Gas turbines and cryogenic air separation processes can be integrated in highly efficient systems for the production of atmospheric gas products. Such systems also can be used to generate electric power in which the gas turbine/air separation system is integrated with a gasification process to generate fuel gas for the gas turbine by the gasification of coal using oxygen from the air separation process. These power generation processes are known as integrated gasification combined cycle (IGCC) processes in which the gas turbine drives an electric generator and the gas turbine exhaust is used to generate steam for a steam turbine which drives another electric generator. In the production of oxygen and/or nitrogen, the pressurized air feed for the cryogenic air separation process can be provided partly or completely by the gas turbine compressor. In both IGCC and air separation systems, nitrogen from the air separation process can be introduced into the gas turbine combustor for additional energy recovery and control of NOx formation, or the nitrogen can be work expanded to drive other process compressors or generate additional electric power.
Comprehensive reviews of integration methods for gas turbines and air separation systems are given in a paper entitled xe2x80x9cNext-Generation Integration Concepts for Air Separation Units and Gas Turbinesxe2x80x9d by A. R. Smith et al in Transactions of the ASME, Vol. 119, Apr. 1997, pp. 298-304 and in a presentation entitled xe2x80x9cFuture Direction of Air Separation Design for Gasification, IGCC, and Alternative Fuel Projectsxe2x80x9d by R. J. Allam et al, IChemE Conference on Gasification, 23-24 Sep. 1998, Dresden, Germany.
A common mode of integration between the gas turbine and air separation units is defined as full air and nitrogen integration. In this operating mode, all air for the gas turbine combustor and the air separation unit is provided by the gas turbine air compressor which is driven by the gas turbine expander, and nitrogen from the air separation unit is utilized in the integrated system. Full air and nitrogen integration is described in representative U.S. Pat. Nos. 3,731,495, 4,224,045, 4,250,704, 4,631,915, and 5,406,786, wherein the nitrogen is introduced into the gas turbine combustor. Full air and nitrogen integration also is described in U.S. Pat. Nos. 4,019,314 and 5,317,862, and in German Patent Publication DE 195 29 681 A1, wherein the nitrogen is work expanded to provide work of compression for the air feed or to generate electric power.
The gas turbine and air separation unit can operate in an alternative mode, defined as partial air integration with full nitrogen integration, in which a portion of the air feed for the air separation unit is provided by the gas turbine compressor and the remainder is provided by a separate air compressor driven by an independent power source. Nitrogen for the air separation unit is introduced into the gas turbine combustor or is otherwise work expanded. This operating mode is described in representative U.S. Pat. Nos. 4,697,415; 4,707,994; 4,785,621; 4,962,646; 5,437,150; 5,666,823; and 5,740,673.
In another alternative, nitrogen integration is used without air integration. In this alternative, the gas turbine and air separation unit each has an independently-driven air compressor, and the nitrogen from the air separation unit is used in the gas turbine combustor. This option is described in representative U.S. Pat. Nos. 4,729,217; 5,081,845; 5,410,869; 5,421,166; 5,459,994; and 5,722,259.
U.S. Pat. No. 3,950,957 and Great Britain Patent Specification 1 455 960 describe an air separation unit integrated with a steam generation system in which a nitrogen-rich waste stream is heated by indirect heat exchange with hot compressed air from the air separation unit main air compressor, the heated nitrogen-rich stream is further heated indirectly in a fired heater, and the final hot nitrogen-rich stream is work expanded in a dedicated nitrogen expansion turbine. The work generated by this expansion turbine drives the air separation unit main air compressor. The nitrogen expansion turbine exhaust and the combustion gases from the fired heater are introduced separately into a fired steam generator to raise steam, a portion of which may be expanded in a steam turbine to drive the air separation unit main air compressor. Optionally, the combustion gases from the fired heater are expanded in a turbine which drives a compressor to provide combustion air to a separate fired heater which heats the nitrogen-rich stream prior to expansion.
An alternative use for high pressure nitrogen from an air separation unit integrated with a gas turbine is disclosed in U.S. Pat. No. 5,388,395 wherein the nitrogen is work expanded to operate an electric generator. The cold nitrogen exhaust from the expander is mixed with the inlet air to the gas turbine compressor thereby cooling the total compressor inlet stream. Alternatively, low pressure nitrogen from the air separation unit is chilled and saturated with water in a direct contact cooler-chiller, and the chilled, saturated nitrogen is mixed with the inlet air to the gas turbine compressor.
U.S. Pat. Nos. 5,040,370 and 5,076,837 disclose the integration of an air separation unit with high-temperature processes which use oxygen, wherein waste heat from the process is used to heat pressurized nitrogen from the air separation unit, and the hot nitrogen is work expanded to generate electric power.
European Patent Publication EP 0 845 644 A2 describes an elevated pressure air separation unit in which the pressurized nitrogen-rich product is heated indirectly by the combustion of low pressure fuel, and the hot nitrogen is work expanded to produce power or drive gas compressors within the air separation unit.
In the production of oxygen in remote areas without accessible electric power grids, feed air compression for an air separation unit can be provided by gas turbine drivers if sufficient fuel is available. In remote areas having industrial operations which require oxygen, inexpensive natural gas often is available and can be used as gas turbine fuel. Industrial operations in such remote areas typically pay a premium for purchased equipment, and therefore simple, reliable equipment is preferred.
An air separation plant integrated with a gas turbine, whether it operates in a remote area or in a populated industrialized area, is subject to various off-design conditions or periods during which the plant operates at lower efficiency or below the design oxygen production rate. These periods occur due to changes in ambient air temperature and/or the cyclic demand for oxygen product. The equipment selection and process design of an integrated air separation plant/gas turbine system therefore must address steady-state operation at design capacity as well as operation at off-design or turndown conditions. This can be difficult to achieve in plants designed for operation in remote locations, particularly in plants with total air integration, because the need for simplified equipment can reduce the number of operating alternatives or degrees of freedom needed for efficient operation at off-design conditions.
The invention disclosed below and defined by the claims which follow addresses the need for improved designs and methods of operation for integrated air separation plant/gas turbine systems, particularly for the operation of such systems in remote areas at off-design or turndown conditions.
The invention is a method for the separation of air which comprises (a) compressing ambient air in a first air compressor to provide a first and a second hot pressurized air feed stream; (b) compressing ambient air in a second air compressor to provide a third hot pressurized air feed stream; (c) combusting fuel with the first hot pressurized air feed stream in a gas turbine combustor, withdrawing therefrom a hot pressurized gas, work expanding the hot pressurized gas in a gas turbine expander, and withdrawing therefrom a gas turbine expander exhaust gas, wherein work produced by the gas turbine expander provides at least a portion of the work required to drive the first and second air compressors; (d) combining the second and third hot pressurized air feed streams, cooling the resulting combined hot pressurized air feed stream, and separating the resulting cooled pressurized air feed stream in an air separation system to yield an oxygen-rich product gas and a nitrogen-rich product gas; and (e) heating and work expanding the nitrogen-rich product gas to yield shaft work and a cooled nitrogen-rich product gas.
Cooling of the resulting combined hot pressurized air feed stream in (d) can be provided at least in part by indirect heat exchange with the nitrogen-rich product gas of (d) and the cooled nitrogen-rich product gas of (e). The method also can further comprise generating steam by indirect heat exchange of the gas turbine expander exhaust gas with water, and introducing the steam into the gas turbine combustor.
The invention can further comprise (f) heating the cooled nitrogen-rich product gas to provide a hot nitrogen-rich product gas, generating steam by indirect heat exchange of the hot nitrogen-rich product gas with water, and introducing the steam into the gas turbine combustor.
The invention can further comprise the combination of (1) heating the cooled nitrogen-rich product gas to provide a hot nitrogen-rich product gas, generating steam by indirect heat exchange of the hot nitrogen-rich product gas with water, and introducing the steam into the gas turbine combustor, and (2) generating steam by indirect heat exchange of the gas turbine expander exhaust gas with water, and introducing the steam into the gas turbine combustor. In addition, if desired, liquid water can be introduced directly into the gas turbine combustor.
The shaft work from work expanding the nitrogen-rich product gas in (e) can be utilized to compress the oxygen-rich product gas.
The cooling of the resulting combined hot pressurized air feed stream in (d) can be provided at least in part by indirect heat exchange with the nitrogen-rich product gas of (d) to yield the hot nitrogen-rich product gas of (f).
In a first alternative embodiment, the invention can further comprise (1) compressing air in a third air compressor to provide a fourth and a fifth hot pressurized air feed stream; and (2) combusting fuel with the fourth hot pressurized air feed stream in an additional gas turbine combustor, withdrawing therefrom an additional hot pressurized gas, expanding the additional hot pressurized gas in an additional gas turbine expander, and withdrawing therefrom an additional gas turbine expander exhaust gas, wherein work produced by the additional expansion turbine is used to compress the oxygen-rich product gas of (d). Steam can be generated by indirect heat exchange of the additional gas turbine expander exhaust gas with water, and the steam can be introduced into the gas turbine combustor of (c). The fifth hot pressurized air feed stream can be combined with the resulting combined hot pressurized air feed stream of (d).
In a second alternative embodiment, the invention can further comprise (1) compressing air in a third air compressor to provide a fourth and a fifth hot pressurized air feed stream; and (2) combusting fuel with the fourth hot pressurized air feed stream in an additional gas turbine combustor, withdrawing therefrom an additional hot pressurized gas, expanding the additional hot pressurized gas in an additional gas turbine expander, and withdrawing therefrom an additional gas turbine expander exhaust gas, wherein work produced by the additional expansion turbine is used to compress the nitrogen-rich product gas of (d). In this alternative embodiment, steam can be generated by indirect heat exchange of the additional gas turbine expander exhaust gas with water, and the steam can be introduced into the gas turbine combustor of (c). The fifth hot pressurized air feed stream can be combined if desired with the resulting combined hot pressurized air feed stream of (d).
In one operating mode of the invention, the temperature of the ambient air increases, thereby decreasing the combined mass flow rate of the first and second hot pressurized air feed streams in (a) and decreasing the mass flow of the third hot pressurized air feed streams in (a). In response to the increase in ambient air temperature, the mass flow rate of the second hot pressurized air feed stream of (a) can be increased such that the mass flow rate of the resulting combined pressurized air feed stream of (d) remains constant. In addition, the mass flow rate of the fuel in (c) can be increased and the mass flow rate of the steam to the gas turbine combustor in (f) can be increased.
In another operating mode of the invention, the temperature of the ambient air increases, thereby decreasing the combined mass flow rate of the first and second hot pressurized air feed streams in (a) and decreasing the mass flow of the third hot pressurized air feed streams in (a). In response to the increase in ambient air temperature, the mass flow rate of the second hot pressurized air feed stream of (a) can be increased such that the mass flow rate of the resulting combined pressurized air feed stream of (d) remains constant. In addition, the mass flow rate of the fuel in (c) can be increased and the mass flow rate of the liquid water to the gas turbine combustor can be increased.
The invention also includes an apparatus for the separation of air which comprises:
(a) a first air compressor to compress ambient air, thereby providing a first and a second hot pressurized air feed stream;
(b) a second air compressor to compress ambient air, thereby providing a third hot pressurized air feed stream;
(c) a gas turbine combustor for combusting fuel with the first hot pressurized air feed stream to yield a hot pressurized gas and a gas turbine expander for work expanding the hot pressurized gas to yield a gas turbine expander exhaust gas, wherein the gas turbine expander is mechanically linked with the first and second air compressors such that work from the gas turbine expander drives the first and second air compressors;
(d) piping means for flow of the first hot pressurized air feed stream from the first air compressor to the gas turbine combustor and for flow of the hot pressurized gas from the gas turbine combustor to the a gas turbine expander;
(e) piping means for combining the second and third hot pressurized air feed streams and providing a resulting combined hot pressurized air feed stream;
(f) cooling means to cool the resulting combined hot pressurized air feed stream to provide a cooled pressurized air feed stream;
(g) an air separation system to separate the resulting cooled pressurized air feed stream into an oxygen-rich product gas and a nitrogen-rich product gas; and
(h) piping means for flow of the cooled pressurized air feed stream from the cooling means to the air separation system.